Studies of the clinical significance of many biomarkers often require large numbers of specimens and many years of patient follow-up. An alternative to completely prospective study designs, which would require years of waiting for answers and would also be prohibitively expensive, is to obtain specimens for which long-term clinical follow-up and considerable correlative clinical data is already available. Nearly 20 years ago the San Antonio breast cancer program began banking such specimens and data for the use of its own investigators. Now, under the SPORE, we have used this experience to develop a Resource for the use of SPORE and other NCI-funded investigators everywhere. We are presently able to provide on a perpetual, self-renewing basis 11,600 specimens per year with important clinical and laboratory data (ER and PgR, S-phase and ploidy), and 1,300 per year with all of these data plus 5-year clinical follow-up. This Resource has already provided specimens for 25 projects, about half of which are outside San Antonio, and has promised support for 8 more. Investigators from all of the other breast cancer SPOREs are included in this support. Specific functions of the Resource include: 1. Receiving new breast cancer specimens and adding them to a computerized inventory system. 2. Maintaining the stored specimens in ultralow temperature freezers. 3. Determining whether the specimens are of sufficient quality for various assays. 4. Maintaining a computerized inventory of tissue in the Resource. 5. Obtaining continual clinical follow-up on a major subset (currently nearly 25,000) of the patients who provided the tumor specimens. 6. Maintaining quality control standards for all data collected. 7. Retrieving banked tissue and distributing the specimens to other NCI- funded investigators.